Plated steel sheet for hot press forming having excellent impact property, hot press formed part, and manufacturing method thereof

ABSTRACT

One aspect of the present invention relates to a plated steel sheet for hot press forming, having an excellent impact property.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/154,177, filed on Jan. 21, 2021, which is a divisional of U.S. patentapplication Ser. No. 16/470,762, filed on Jun. 18, 2019, now patented asU.S. Pat. No. 10,934,601, issued on Mar. 2, 2021, which is the U.S.National Phase under 35 U.S.C. § 371 of International Patent ApplicationNo. PCT/KR2017/014843, filed on Dec. 15, 2017, which claims priority toKorean Patent Application No. 10-2016-0178236, filed on Dec. 23, 2016,the disclosures of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present disclosure relates to a plated steel sheet for hot pressforming, a hot press formed part, and manufacturing method thereof,having excellent impact properties, applicable to automotive componentsrequiring impact resistance properties.

BACKGROUND ART

Recently, a hot press formed part, having high strength, has beenapplied to automotive structural members to achieve improvements in fuelefficiency, protection of passengers, and like this, through weightreduction of automobiles.

A representative technology, relating to such a hot press moldingmember, is disclosed in Patent Document 1. According to Patent Document1, an Al—Si plated steel sheet is heated to 850° C. or higher and thenhot press formed and quenched by a press, and a structure of the membercan be formed into martensite. Therefore, it is possible to secure anultra-high strength property with a tensile strength of 1600 MPa ormore, thereby easily achieve a light weight of the automobile.

However, according to Patent Document 1, an impact property againstimpact is relatively deteriorated due to high strength. In certaincases, an abnormally low impact property is exhibited depending on a hotpress forming condition. Accordingly, there was an increasing demand fordevelopment of a hot press formed member having excellent impactproperties.

Patent Document 2 proposes a technology to improve an impact propertyafter hot press forming, which is achieved by adjusting a ratio ofcalcium to sulfur (Ca/S) to spheroidize an inclusion and adding analloying element such as niobium (Nb) to achieve grain refinement.

However, Patent Document 2, disclosing inclusion control and grain sizecontrol for improving an impact property of a typical iron material, isestimated to have difficulty in improving a low impact propertyoccurring in actual hot press forming.

Accordingly, there is a demand for development of a plated steel sheetfor hot press forming, a hot press formed part, and manufacturing methodthereof, having excellent impact properties.

PRIOR ART DOCUMENT

-   (Patent Document 1) US Patent Publication No. 6296805-   (Patent Document 2) Korea Patent Publication No. 10-2010-0047011

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a plated steel sheetfor hot press forming having excellent impact properties, a hot pressformed part using the plated steel sheet for hot press forming, and amanufacturing method thereof.

Aspects of the present disclosure are not limited to the above-mentionedaspects. The above-mentioned aspects and other aspects of the presentdisclosure will be clearly understood by those skilled in the artthrough the following description.

Technical Solution

According to an aspect of the present disclosure, a plated steel sheetfor hot press forming, having an excellent impact property, comprises abase steel sheet, and an Al—Si plated layer formed on the surface of thebase steel sheet. The thickness of a carbon-depleted layer in a surfacelayer part of the base steel sheet is 5 μm or less. The surface layerpart means a region from the surface of the base steel sheet to a depthof 200 μm, and the carbon-depleted layer means a region which the carboncontent is 50% or less of an average carbon amount (C₀) of the basesteel sheet.

According to another aspect of the present disclosure, a manufacturingmethod of a plated steel sheet for hot press forming, having excellentimpact properties, comprises heating a slab, to a temperature of 1050 to1300° C., performing finishing hot-rolling on the heated slab to atemperature of 800 to 950° C. to obtain a hot-rolled steel sheet,stating cooling the hot-rolled steel sheet within 30 seconds ofperforming the finishing hot-rolling and winding the hot-rolled steelsheet at a temperature of 450 to 750° C., heating the wound hot-rolledsteel sheet to a temperature of 740 to 860° C. and annealing in anatmosphere having a dew point temperature of −70 to −30° C., and platingthe annealed hot-rolled steel sheet by dipping in an Al—Si plating bath.

According to an aspect of the present disclosure, a hot press formedpart comprises a base material, and an Al—Si plated layer formed on asurface of the base material. A carbon-enriched layer is formed in asurface layer part of the base material. The surface layer part means aregion from the surface of the base material to a depth of 200 μm, andthe carbon-enriched layer means a region which the carbon content is110% or more of an average carbon amount (C₀) of the base material.

According to another aspect of the present disclosure, a manufacturingmethod of a hot press formed part comprises a heating a plated steelsheet, manufactured by the manufacturing method of a plated steel sheetfor hot press forming according to the present disclosure, to atemperature ranging from an Ac3 temperature to 980° C. and maintainingfor 1 to 1000 seconds, and a hot press forming the heated plated steelsheet by a press while cooling a martensite transformation finishtemperature (Mf) or less at a cooling rate of 1 to 1000° C./sec.

In addition, the above-mentioned technical solution does not list allthe features of the present disclosure. The various features of thepresent disclosure, advantages and effects thereof can be understood inmore detail with reference to the following specific embodiments.

Advantageous Effects

Asset forth above, example embodiments in the present disclosure mayprovide a plated steel sheet for hot press forming, a hot press formedpart using the plated steel sheet for hot press forming, andmanufacturing method thereof, having excellent impact properties.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating TSxIE values depending on thicknesses ofC thickening layers of base material surface layer parts of hot pressformed parts according to an example embodiment in the presentdisclosure.

FIG. 2 is images obtained by capturing microstructures after performingnital etching on wall portions of hot press formed parts A1-1 (inventionexample) and A2-1 (comparative example) among examples.

FIG. 3 is graphs illustrating concentration profiles of main componentsthrough GDS analysis before and after hot press forming of A1-1(invention example).

BEST MODE FOR INVENTION

Hereinafter, example embodiments in the present disclosure will bedescribed. However, the embodiments in the present disclosure may bemodified to various other forms, and the scope of the present disclosureis not limited to the embodiments described below. In addition, theembodiments in the present disclosure are provided in order to morecompletely describe the present disclosure for those having averageknowledge in the art.

The present inventors have found that a hot press formed part accordingto a related art has a poor impact property, and have recognized thatinclusion control and grain size control for improving an impactproperty of a typical steel material are limited in improving a lowimpact property occurring in actual hot process forming. To address theissues, the present inventors conducted a profound research.

A low impact property, occurring in actual hot press forming, is causedby presence of locally formed ferrite of a surface layer part. When suchferrite is locally formed, deformation, resulting from applied impact,is concentrated on the ferrite to be easily fractured.

Such a phenomenon is significantly important in an actual component. Theactual component has a complex shape and has a flat portion, being infull contact with a mold perpendicularly to a moving direction of themold, and a wall portion being in contact with the mold horizontally tothe moving direction of the mold or obliquely at a small angle. In thecase of such a wall portion, a contact with the mold is insufficientdepending on a component shape or phase transformation is promotedaccording to hot press forming to establish a condition in which ferriteis easily formed on a surface layer of a base material.

Therefore, the present inventors concluded that an impact propertydeterioration factor such as surface ferrite, involved in hot pressforming, should be significantly reduced to manufacture a hot pressforming part having excellent impact properties. To this end, thepresent inventors concluded that hardenability needed to be locallyimproved in a surface layer portion. Accordingly, the present inventorconfirmed that a carbon-enriched layer was formed on a base materialsurface layer in the hot press forming to provide a plated steel sheetfor hot press forming, a hot press formed part using the plated steelsheet for hot press forming, and a manufacturing method of thereof,having excellent impact properties. For these reasons, the inventorsconceived the present invention.

Plated Steel Sheet for Hot Press Forming Having Excellent ImpactProperty

A plated steel sheet for hot press forming, having excellent impactproperties, according to an aspect of the present disclosure will bedescribed.

Hereinafter, a plated steel sheet for hot press forming having excellentimpact properties according to one aspect of the present invention willbe described in detail.

The plated steel sheet for hot press forming, having excellent impactproperties, according to one aspect of the present disclosure includesabase steel sheet, comprising, by weight percent (wt %), 0.15 to 0.4% ofcalcium (C), 0.05 to 1.0% of silicon (Si), 0.6 to 3.0% of manganese(Mn), 0.001 to 0.05% of phosphorus (P), 0.0001 to 0.02% of sulfur (S),0.01 to 0.1% of aluminum (A1), 0.001 to 0.02% of nitrogen (N), 0.001 to0.01% of boron (B), 0.01 to 0.5% of chromium (Cr), 0.01 to 0.05% oftitanium (Ti), and a balance of iron (Fe) and inevitable impurities, andan Al—Si plated layer disposed on a surface of the base steel sheet. Acarbon-depleted layer has a thickness of 5 micrometers (μm) or less on asurface layer part of the base steel sheet (the surface layer part meansa region from a surface of the base steel sheet to a depth of 200 μm,and the carbon-depleted layer means a region which the carbon content is50% or less of an average C amount (C0) of the base steel sheet).

First, an alloy composition of commonly applied to a base steel sheet ofthe plated steel sheet, a base material of the hot press formed part,and a slab of a producing method, will be described in detail.Hereinafter, a unit of the content of each component will be weightpercent (wt %) unless otherwise specified.

Carbon (C): 0.15 to 0.4%

Carbon (C) is an element essential to improve strength of an hot pressformed part.

When a content of carbon (C) is less than 0.15%, it is difficult tosecure sufficient strength. Meanwhile, when the content of carbon (C) isgreater than 0.4%, strength of a hot-rolled material is so high that acold rolling property may be significantly deteriorated duringcold-rolling of the hot-rolled material, and spot weldability may besignificantly reduced. Accordingly, the content of carbon (C) is, indetail, 0.15 to 0.4%.

In addition, a lower limit of the content of carbon (C) may be, infurther detail, 0.18% and an upper limit of the content of carbon (C)may be, in detail, 0.35%.

Silicon (Si): 0.05 to 1.0%

Silicon (Si) needs to be added a deoxidizer and serves as a solidsolution strengthening element and a carbide formation suppressingelement in steel making. In addition, silicon (Si) contributes toimprovement in the strength of a hot press formed part and is effectivein material uniformity.

When a content of silicon (Si) is less than 0.05%, the above-mentionedeffect is insufficient. Meanwhile, when the content of silicon (Si) isgreater than 1.0%, aluminum (A1) platability may be significantlyreduced by a silicon (Si) oxide formed on a surface of a steel sheetduring annealing. Therefore, the content of silicon (Si) is, in detail,0.05 to 1.0%.

In addition, a lower limit of the content of silicon (Si) may be, infurther detail, 0.08%, and an upper limit of the content of silicon (Si)may be, in further detail, 0.9%.

Manganese (Mn): 0.6 to 3.0%

Manganese (Mn) needed to be added to secure a solid solutionstrengthening effect and to reduce a critical cooling rate for securingmartensite in the hot press formed part.

When a content of manganese (Mn) is less than 0.6%, there is alimitation in obtaining the above effect. Meanwhile, when the content ofmanganese (Mn) is greater than 3.0%, a cold rolling property is reduceddue to an increase in strength of a steel sheet before a hot processforming process, and the cost of a ferro-alloy is increased and spotweldability is deteriorated. Accordingly, the content of manganese (Mn)is, in detail, 0.6 to 3.0%.

In addition, a lower limit of the content of manganese (Mn) may be, infurther detail, 0.8% and an upper limit of the content of manganese (Mn)may be, in further detail, 2.8%.

Phosphorus (P): 0.001 to 0.05%

Phosphorus (P) is an impurity. High manufacturing costs are incurred tocontrol a content of phosphorus (P) to be less than 0.001%. When thecontent of phosphorous (P) is greater than 0.05%, weldability of the hotpress formed part is significantly deteriorated. Accordingly, thecontent of phosphorous (P) is, in detail, 0.001 to 0.05%.

Sulfur (S): 0.0001 to 0.02%

High manufacturing costs are incurred to control the content of sulfur(S) to be less than 0.0001%. When the content of sulfur (S) is greaterthan 0.02%, ductility, an impact property, and weldability of the hotpress formed part are deteriorated. Accordingly, the content of sulfur(S) is, in detail, 0.0001 to 0.02%.

Aluminum (A1): 0.01 to 0.1%

Aluminum (A1) is an element performing a deoxidation action togetherwith silicon (Si) in steelmaking to improve cleanliness of steel.

When a content of aluminum (A1) is less than 0.01%, the above-mentionedeffect is insufficient. When the content of aluminum (A1) is greaterthan 0.1%, high-temperature ductility is reduced due to an excessivealuminum nitride (AlN) formed during a continuous casting process, andslab cracking is apt occur.

Nitrogen (N): 0.001 to 0.02%

Nitrogen (N) is included in the steel as an impurity. High manufacturingcosts are incurred to control a content of nitrogen (N) to be less than0.001%. When the content of nitrogen (N) is greater than 0.02%,high-temperature ductility is reduced due to an excessive aluminumnitride (AlN) formed during a continuous casting process, and slabcracking is apt to occur.

Boron (B): 0.001 to 0.01%

Boron (B) is an element which may improve hardenability even if a smallamount of boron (B) is added, and is an element which may segregatealong prior-austenite grain boundaries to suppress embrittlement of ahot press formed part caused by boundary segregation of phosphorus (P)and/or sulfur (S). However, when the content of boron (B) is less than0.0001%, it is difficult to obtain such an effect. When the content ofboron (B) is greater than 0.01%, such an effect may be saturated and mayresult in brittleness at hot rolling.

Chromium (Cr): 0.01 to 0.5%

Chromium (Cr) is added to secure the hardenability of the steel such asmanganese (Mn), boron (B), or the like.

When a content of chromium (Cr) is less than 0.01%, it is difficult tosecure sufficient hardenability. When the content of chromium (Cr) isgreater than 0.5%, the hardenability may be sufficiently secured, butcharacteristics thereof may be saturated and steel sheet producing costsmay be increased.

Titanium (Ti): 0.01 to 0.05%

Ti is added in order to retain the solidified boron (B) which isessential for securing the hardenability. This is because Ti is combinedwith nitrogen remained in the steel as an impurity to form TiN.

When a content of titanium (Ti) is less than 0.01%, the above-mentionedeffect is insufficient. When the content of titanium (Ti) is greaterthan 0.05%, the characteristics may be saturated and steel sheetproducing costs may be increased.

In the present disclosure, the other component is iron (Fe). However,impurities in raw materials or manufacturing environments may beinevitably included, and such impurities may not be excluded. Since suchimpurities are well to a person of ordinary skill in manufacturingindustries, descriptions thereof will not be given in the presentdisclosure.

In addition to the above-described alloy composition, at least oneselected from molybdenum (Mo), niobium (Nb), and vanadium (V) may beadditionally contained in such a manner that a total amount is set to be0.01 to 0.5 wt %.

Mo, Nb and V are elements contributing to improvement in strength andincrease in impact toughness caused by grain refinement. When the totalamount thereof is less than 0.01%, the above-mentioned effect may not beobtained. When the total amount is greater than 0.5%, the effect may besaturated and the manufacturing costs may be increased.

In the plated steel sheet for hot press forming according to the presentdisclosure, a thickness of a carbon-depleted layer is 5 micrometers (μm)or less on a surface layer part of a base steel sheet. The surface layerpart means a region from the surface of the base steel sheet to a depthof 200 μm, and the carbon-depleted layer means a region which the carboncontent is 50% or less of an average C amount (C0) of the base steelsheet.

When the thickness of the carbon-depleted layer in the surface layerpart of the base steel sheet is greater than 5 μm, it is difficult tosufficiently form the carbon-enriched layer in the surface layer part ofthe base material after the final hot press forming. Therefore, thethickness of the carbon-depleted layer may be, in detail, 5 μm or lessand, in further detail, 4 μm or less.

In the base steel sheet, a ratio of a carbide fraction of the surfacelayer part (Fs) to a carbide fraction of a central portion (Fc) (Fs/Fc)may be 0.7 to 1.3. The surface layer part means a region from thesurface of the base steel sheet to a depth of 200 μm, and the centralportion means a region having a thickness of −100 μm to +100 μm from athickness center of the base steel sheet.

A carbide, present in the base steel sheet, is dissolved during hotpress forming to supply carbon. When the ratio Fs/Fc is less than 0.7,it is difficult to sufficiently form a carbon-enriched layer on asurface layer of the base material during the hot press forming.Meanwhile, when the Fs/Fc is greater than 1.3, the carbon-enriched layermay be sufficiently formed. However, before annealing, a specialtreatment such as a carburizing treatment is required to control theFs/Fc to be greater than 1.3. Moreover, manufacturing costs isincreased.

A microstructure of the base steel sheet is not limited, but mayinclude, by an area fraction, for example, 10 to 40% of pearlite, 50 to90% of ferrite, and 20% or less of martensite.

The plated steel sheet for hot process forming according to the presentdisclosure comprises an Al—Si plated layer formed on the surface of thebase steel sheet. The Al—Si plated layer suppresses surfacedecarburization during hot press forming and serves to improve corrosionresistance.

The plated layer may contain, by weight percent (wt %), 6 to 12% ofsilicon (Si), 1 to 4% of iron (Fe), and a balance of aluminum (A1) andinevitable impurities. This is because a composition of a galvanizingbath is not limited but needs to be controlled, as set forth below. Aplated layer, having a composition nearly the same as the composition ofthe plating bath, is formed and has a higher content of iron (Fe) thanthe plating bath, but satisfies the above range.

When a content of silicon (Si) is less than 6%, fluidity of the platingbath is reduced to makes it difficult to uniformly form a plated layer.On the other hand, when the content of silicon (Si) is greater than 12%,a melting temperature of the plating bath is increased, and thus amanagement temperature of the plating bath should be increased. Iron(Fe) in the plating bath is dissolved to be present in the plating bathfrom the steel sheet during a plating process. In order to maintain acontent of iron (Fe) in the galvanizing bath at less than 1%, excessivemanufacturing costs are incurred to dilute iron (Fe) dissolved anddischarged. When the content of iron (Fe) is greater than 4%, analuminum iron (FeAl) compound, dross, is apt to be formed in the platingbath to degrade plating quality. Therefore, the content of iron (Fe)needs to be controlled at 4% or less.

The plated layer may have a thickness of 10 to 45 μm.

When the plated layer has a thickness less than 10 μm, it is difficultto secure the corrosion resistance of the hot press formed part. Whenthe plated layer has a thickness greater than 45 μm, the manufacturingcosts may be increased due to excessive plating adhesion amount, and itmay be difficult to uniformly plate the steel sheet in width and lengthdirection a coil.

Manufacturing Method of Plated Steel Sheet for Hot Press Forming HavingExcellent Impact Property

Hereinafter, a manufacturing method of a plated steel sheet for hotpress forming having excellent impact properties, another aspect of thepresent disclosure, will be described in detail.

The manufacturing method comprises heating a slab, satisfying theabove-mentioned alloy composition, to a temperature of 1050 to 1300° C.,performing finishing hot-rolling on the heated slab to a temperature of800 to 950° C. to obtain a hot-rolled steel sheet, stating cooling thehot-rolled steel sheet within 30 seconds of performing the finishinghot-rolling and winding the hot-rolled steel sheet at a temperature of450 to 750° C., heating the wound hot-rolled steel sheet to atemperature of 740 to 860° C. to be annealed in an atmosphere having adew point temperature of −70 to −30° C., and plating the annealedhot-rolled steel sheet by dipping in an aluminum-silicon (Al—Si) platingbath.

Slab Heating

The slab, satisfying the above-mentioned alloy composition, is heated to1050 to 1300° C.

When a slab heating temperature is lower than 1050° C., it may bedifficult to homogenize a slab structure. When the slab heatingtemperature is higher than 1300° C., an excessive oxidation layer isformed and a surface layer part is severely decarburized during heating.Therefore, it is difficult to form a carbon-enriched layer on thesurface layer part of a base material during ultimate hot press forming,as intended in the present disclosure.

Hot-Rolling

The heated slab is hot-rolled at 800 to 950° C. to obtain a hot-rolledsteel sheet.

When a finishing hot-rolling temperature is lower than 800° C., it isdifficult to control a plate shape, resulting from generation of a mixedgrain structure in the surface layer part of the steel sheet dependingon rolling at two phase regions. When the finishing hot-rollingtemperature is higher than 950° C., coarse grains may be formed.

Cooling and Winding

The hot-rolled steel sheet starts to be cooled within 30 seconds offinishing hot rolling, and is wound at 450 to 750° C.

When the cooling is started over 30 seconds, surface decarburization isaccelerated as high temperature maintenance time is increased, and acarbon-depleted layer is formed in the surface layer part. Even afterhot press forming, such a carbon-depleted layer remains in the basematerial to make it difficult to form a carbon-enriched layer.

When the winding temperature is lower than 450° C., it is difficult tocontrol a plate shape because martensite is formed in the whole or aportion of the steel sheet and a cold rolling property may bedeteriorated due to an increase in strength of the hot-rolled steelsheet. Meanwhile, when the winding temperature is higher than 750° C.,surface decarburization is accelerated and surface quality is degradeddue to internal oxidation after subsequent plating.

Annealing

The wound hot-rolled steel sheet is heated to 740 to 860° C. andannealed in an atmosphere having a dew point temperature of −70 to −30°C.

When the annealing temperature is less than 740° C., the cold-rolledstructure is insufficiently recrystallized to cause a poor plate shapeor strength following plating is excessively increased to cause wear ofa mold during a blanking process. On the other hand, when the annealingtemperature is higher than 860° C., a surface oxide such as silicon(Si), manganese (Mn), or the like is formed during an annealing processto cause a poor Al—Si plated surface.

Additional equipment for controlling a composition of gas or the like isrequired to control a dew point temperature of the atmosphere to be lessthan −70° C., and increases manufacturing costs. When the dew pointtemperature is higher than −30° C., a surface of the steel sheet isdecarburized during annealing to make it difficult to form acarbon-enriched layer on a surface layer after an ultimate heattreatment, as intended in the present disclosure.

In this case, the annealing process may be performed in a non-oxidizingatmosphere, for example, a hydrogen-nitrogen mixed gas may be used.

The manufacturing method may further comprise cold-rolling the woundhot-rolled steel sheet to obtain a cold-rolled steel sheet before theannealing.

Although annealing and plating may be performed without performing coldrolling, the cold rolling may be performed to more precisely control athickness of the steel sheet. The cold rolling may be performed at areduction ratio of 30 to 80%.

Plating

The wound hot-rolled steel sheet is plated by dipping in an Al—Siplating bath. When the cold rolling and the annealing are performed, theannealed cold-rolled steel sheet is plated by dipping in the Al—Siplating bath.

In this case, the Al—Si plating bath may comprise, by weight percent (wt%), 6 to 12% of silicon (Si), 1 to 4% of iron (Fe), and a balance ofaluminum (A1) and inevitable impurities.

When a content of silicon (Si) is less than 6%, fluidity of the platingbath is reduced to make it difficult to form a uniform plated layer. Onthe other hand, when the content of silicon (Si) is greater than 12%,the melting temperature of the plating bath is increased, and thus theplating bath management temperature should be increased. Iron (Fe) inthe plating bath is dissolved to be present in the plating bath from thesteel sheet during the plating process. In order to maintain a contentof iron (Fe) in the plating bath at less than 1%, excessivemanufacturing costs are incurred to dilute iron (Fe) dissolved anddischarged. When the content of iron (Fe) is greater than 4%, analuminum iron (FeAl) compound, dross, is apt to be formed in the platingbath to degrade plating quality. Therefore, the content of iron (Fe)needs to be controlled at 4% or less.

The plating may be performed in such a manner that plating weight is setto be 30 to 130 g/m2 on the basis of one side.

When the plating weight is less than 30 g/m2 on the basis of one side,it is difficult to secure the corrosion resistance of the hot pressformed part. When the plating weight is greater than 130 g/m2, themanufacturing costs may be increased due to excessive plating adhesionamount, and it may be difficult to uniformly plate the steel sheet inwidth and length direction a coil.

Hot Press Formed Part Having Excellent Impact Property

Hereinafter, a hot press formed part having excellent impact properties,another aspect of the present invention, will be described in detail.

The hot press formed part comprises a base material, satisfying theabove-described alloy composition, and an aluminum-silicon (Al—Si)plated layer formed on a surface of the base material. A carbon-enrichedlayer is formed on the surface layer part of the base material.

The surface layer part means a region from the surface of the basematerial to a depth of 200 μm, and the carbon-enriched layer means aregion which the carbon content is 110% or more of an average C amount(C₀) of the base material.

The carbon-enriched layer is provided to improve hardenability of thesurface layer part, and suppresses the formation of ferrite in thesurface layer to improve the impact property.

In this case, the carbon-enriched layer may have a thickness of to 150μm.

When the thickness of the carbon-enriched layer is less than 10 μm, aneffect of suppressing formation of ferrite in the surface layer part anda hardenability improving effect are insufficient. Thus, ultimately, theimpact property of the hot press formed part may be deteriorated. Inorder to control the thickness of the carbon-enriched layer to begreater than 150 μm, a heat treatment needs to be performed for a longperiod of time, and additional manufacturing costs such as carburizingatmosphere control may be increased.

A microstructure of the base material is not limited. For example, themicrostructure of the base material may contain, by area fraction, 90%or more of martensite or bainite as a main phase, 10% or less offerrite, and 5% or less of residual austenite.

In this case, the hot press formed part may have tensile strength (TS)of 1300 MPa or more and impact absorption energy (IE) of 4.0 J measuredusing a sample having a thickness of 1.5 mm at 25° C.

The hot press formed part may have a product of tensile strength (TS)and impact absorption energy (IE), measured using a sample having athickness of 1.5 mm at a temperature of 25° C., (TSxIE) greater than orequal to 8000 MPa·J.

Manufacturing Method of Hot Press Formed Part Having Excellent ImpactProperty

Hereinafter, a manufacturing method of a hot press formed part havingexcellent impact properties, another aspect of the present invention,will be described in detail.

The manufacturing method comprises a heating a plated steel sheet,manufactured by the above-described manufacturing method of a platedsteel sheet according to the present disclosure, to a temperatureranging from an Ac3 temperature to 980° C. and maintaining for 1 to 1000seconds, and a hot press forming the heated plated steel sheet by apress while cooling to a martensite transformation finish temperature(Mf) or less at a cooling rate of 1 to 1000° C./sec.

Heating A Plated Steel Sheet

The plated steel sheet, manufactured by the above-describedmanufacturing method of a plated steel sheet according to the presentdisclosure, is heated to a temperature ranging from an Ac3 temperatureto 980° C. and maintained for 1 to 1000 seconds.

When the heating temperature is lower than the Ac3 temperature, thepresence of untransformed ferrite makes it difficult to securepredetermined strength. When the heating temperature is higher than 980°C., excessive formation of an oxide on the part surface makes itdifficult to secure spot weldability.

When the maintenance time is less than 1 second, the temperature may notbe uniform and some carbides may be insufficiently redissolved, whichmay cause material differences in each portion. When the maintenancetime is greater than 1000 seconds, an oxide is excessively formed on apart surface to make it difficult to secure spot weldability, similarlyto the case of the higher heating temperature.

In this case, the total heating rate during heating is not limited butmay be set in such a manner a heating rate of 600 to 800° C. is 1 to 10°C./sec to more easily form the carbon-enriched layer in a surface layerof the base material.

In general, the presence of an Al—Si plated layer suppressesdecarburization of the base material during heating. However, thetemperature ranging from 600 to 800° C. is a range in which the Al—Siplated layer is melted during heating and a liquid plated layer ispresent, and a continuous decarburization reaction occurs. Accordingly,when the heating rate is lower than 1° C./sec in the temperature rangingfrom 600 to 800° C., the continuous decarburization reaction makes itdifficult to ultimately form a sufficient carbon-enriched layer in thesurface layer part of the base material. When the heating rate isgreater than 10° C./sec in the temperature ranging from 600 to 800° C.,the continuous decarburization reaction may be significantly reduced,but additional heating equipment is required to increase themanufacturing costs.

Hot Press Forming

The manufacturing method comprises a hot press forming the heated platedsteel sheet by a press while cooling to a martensite transformationfinish temperature (Mf) or less at a cooling rate of 1 to 1000° C./sec.

When the cooling rate is less than 1° C./sec, ferrite is formed to makeit difficult to secure high strength. In order to control the coolingrate to be higher than 1000° C./sec, expensive special cooling equipmentis required to increase the manufacturing costs.

MODE FOR INVENTION

Hereinafter, example embodiments in the present disclosure will bedescribed in detail. However, the present disclosure is not limited tothe example embodiments described herein.

A slab, having a thickness of 40 mm and a composition listed in Table 1,was prepared by vacuum melting.

The slab was heated, hot rolled, cooled, and wound under the conditions,listed in Table 2, to produce a hot-rolled steel sheet having athickness of 3 mm. A plated steel sheet was produced by pickling thehot-rolled steel sheet, cold-rolling the pickled steel sheet at areduction ratio of 50%, annealing the cold-rolled steel sheet in anatmosphere of 5% of hydrogen and 95% of nitrogen under the conditionslisted in Table 2, and plating the annealed steel sheet by dipping in aplating bath containing aluminum (A1), 9% of silicon (Si), and 2% ofiron (Fe) in such manner that a plating amount is 80 g/m2 on the basisof one side.

In a base steel sheet of the plated steel sheet, a thickness of acarbon-depleted layer of a surface layer part, a carbide fraction Fc ofa central portion, and a surface layer carbide fraction Fs of a surfacelayer part carbide were observed and measured and listed Table 2. Thecarbon depletion layer means a region which the carbon content is 50% orless of an average C amount (C₀) of the base steel sheet, the surfacelayer part means a region which 200 μm from a surface of the base steelsheet, and the central portion means a region which is −100 μm to +100μm from a thickness center of the base steel sheet.

In distribution analysis of carbon, concentration analysis on mainelements such as carbon, and the like, was performed with respect tosufficient depth from a surface layer by using a glow dischargespectrometer (GDS) method which quantitative analyzes various componentsin the depth direction. In the case of a typical GDS analysis, it isdifficult to precisely specify a boundary between a plated layer and abase material due to a depth profile for the depth direction because adepth-direction analysis is performed on a circular area of 2 to 6 mm.However, based on various optical and SEM analysis results, a point, atwhich the content of aluminum (A1) is 1%, was referenced as a basematerial surface layer.

A carbide of the base steel sheet was measured by an image analyzerafter 1000-magnification SEM observation is performed in three locationsof each of the surface layer part and the central portion.

Hot press forming was performed to manufacture a hot press formed partby heating the plated steel sheet under heating conditions listed inTable 3 and transferring the heated steel sheet to a press in which ahat-shaped mold is seated. Time, from heating furnace extraction tostart of the hot press forming, was 12 seconds and equivalently applied.Samples were taken out of a wall portion which is vulnerable toformation of ferrite in an actual component, and a thickness, yieldstrength (YS), and tensile strength (TS), elongation (EL), and impactabsorption energy (IE) of the base material surface layer part wereobserved and measured and listed in Table 3.

Before charging into the heating furnace, a thermocouple was attached toa central portion of a blank to measure heating behaviors of a steelsheet. Then, a heating rate in a temperature section of 600 to 800° C.was measured based on the heating behaviors.

The carbon thickening layer means a region which the carbon content is110% or more of an average C amount (C₀) of the base material, and thethickness of the carbon-enriched layer in the base material surfacelayer was measured using GDS analysis, which is the same method asanalysis of the carbon-depleted layer.

A Charpy impact toughness test was performed to evaluate an impactproperty. In the present disclosure, each of the steel sheets was a thinmaterial having a thickness of 1.5 mm, and it was difficult to prepare aCharpy impact toughness sample having a standard size. Therefore, ineach test, a V-notch type sample, having only a size reduced to 1.5 mmcompared with the standard size, was prepared and evaluated at a roomtemperature (25° C.), and impact energy (IE) was measured. Since atypical impact property is inversely proportional to strength, theimpact property was expressed together with a result converted intoTSxIE. Classification was made based on the result of TSxIE to confirmexcellent impact properties intended in the present disclosure.

TABLE 1 Type of Note Steel C Si Mn P S Al N B Cr Ti Other IS A 0.23 0.21.3 0.01 0.002 0.03 0.004 0.0025 0.21 0.035 IS B 0.31 0.5 1 0.009 0.0010.028 0.0043 0.0027 0.4 0.032 IS C 0.18 0.8 2.5 0.011 0.003 0.031 0.00380.0022 0.15 0.037 IS D 0.25 0.1 1.1 0.009 0.002 0.029 0.0045 0.0024 0.20.032 Mo:0.1 IS E 0.25 0.1 1.13 0.009 0.002 0.032 0.003 0.0023 0.190.025 Nb:0.06 V:0.1 CS F 0.12 0.2 1.5 0.011 0.002 0.03 0.005 0.0025 0.220.033 *IS: Inventive Steel *CS: Comparative Steel

TABLE 2 Slab Finishing Thickness of Plated Heating Rolling CoolingWinding Carbon-Depleted Steel Type of Temperature Temperature StartingTemperature Annealing Condition Layer Carbide Sheet Steel (° C.) (° C.)(Sec) (° C.) Heating Dew Point (μm) Fs Fc Fs/Fc A1 A 1200 880 10 600 780−50  0 16.4 18.1 0.9  A2 1200 900 10 650 780    0 22 11.4 19.3 0.59 A31200 900 10 650 780 −20  4 11.3 18.6 0.61 A4 1370 900 10 650 780 −45 1212.8 17.8 0.72 A5 1200 880 45 600 820 −45  7 12.5 18.4 0.68 B1 B 1200880 10 600 780 −40  0 31.6 36.7 0.86 C1 C 1250 900  5 500 800 −55  016.5 15.4 1.07 C2 1250 900  5 500 800 −35  3 15.2 16.7 0.91 D1 D 1300920 10 550 760 −40  0 21.3 19.2 1.11 E1 E 1300 920 10 550 760 −40  021.9 20.9 1.05 F1 F 1200 880 10 600 780 −50  0 13.2 13.8 0.96

TABLE 3 Heating condition Hot Mainte- Press Plated nance Heating Carbon-Mechanical Properties Molding Steel Tempera- Time Rate enriched YS TS ELIE TSxIE Member Sheet ture (Min) (° C./S) layer (MPa) (MPa) (°) (J) (MPa· J) Note A1-1 A1 900 6 3   40 1065 1499 6 8.6 12895 IE A1-2 930 8 3.472 1032 1481 7 9.3 13771 IE A1-3 930 3 3.4 51 1062 1498 7 9.2 13785 IEA1-4 900 6 2   21 1053 1489 6 7.5 11165 IE A1-5 900 6 0.5   7.2 10281465 7 3.1  4542 CE A2-1 A2 900 6 3    0 983 1361 7 1.2  1633 CE A3-1 A3900 6 3    0 991 1399 7 1.9  2659 CE A4-1 A4 910 5 3.1  0 998 1391 6 1.4 1947 CE A5-1 A5 910 5 3.1  0 1002 1395 7 2.1  2929 CE B1-1 B1 850 5 2.614 1250 1821 6 5.6 10198 IE B1-2 880 5 2.9 22 1281 1837 5 5.3  9736 IEB1-3 900 5 4.5 28 1285 1842 5 4.9  9026 IE C1-1 C1 900 6 3   43 971 13518 8.8 11889 IE C1-2 950 8 8.5 86 957 1329 8 9.9 13157 IE C2-1 C2 850 30.4   4.7 954 1312 8 3.4  4461 CE D1-1 D1 900 6 5   43 1144 1577 6 8.713720 IE E1-1 E1 900 6 5   55 1151 1569 6 8.5 13337 IE F1-1 F1 900 6 3  34 851 1208 8 10.3 12442 CE *IE: Invention example *CE: ComparativeExample

In the case of the invention examples satisfying conditions of thepresent disclosure, excellent tensile strength and impact propertiescould be secured.

Meanwhile, in the case of plated steel sheets A2 and A3 in which dewpoint temperatures of an atmosphere during annealing were out of therange proposed in the present disclosure, a depth of a carbon-depletedlayer was greater than 5 μm or Fs/Fc is less than 0.7 according tosurface decarburization during the annealing. In hot press formed partsA2-1 and A3-1, manufactured using such characteristics, acarbon-enriched layer of a base material surface layer part had athickness less than 10 μm, and impact properties were deteriorated.

In the case of a plated steel sheet A4, a slab heating temperature wasout of the range proposed in the present disclosure. In the case of aplated steel sheet A5, a cooling start time following finishing rollingwas out of the range of the present disclosure. In each of the platedsteel sheets A4 and A4, a carbon-depleted layer had a depth of 5 μm orFs/Fc was less than 0.7. In hot press formed parts A4-1 and A5-1,manufactured using such characteristics, a carbon-enriched layer of abase material surface layer part was not formed, and impact propertieswere deteriorated.

In hot press forming parts A1-5 and C2-1 were manufactured using platedsteel sheets A1 and C2 satisfying the conditions proposed in the presentdisclosure, a heating rate in a temperature section of 600 to 800° C.was 1° C./sec, which was out of the range proposed in the presentdisclosure. In each of hot press forming parts A1-5 and C2-1, acarbon-enriched layer of a base material surface layer part wasinsufficiently formed to have a thickness of 10 μm or less, and animpact property was deteriorated.

In the case of a steel F, a content of carbon (C) was low. It can beseen that a hot press formed part F1-1 had deteriorated tensilestrength.

FIG. 1 is a graph illustrating TSxIE values depending on thicknesses ofC thickening layers of base material surface layer parts of hot pressmolding members according to an example embodiment in the presentdisclosure. From the graph in FIG. 1 , it can be seen that the TSxIEvalue was significantly increased when the carbon-enriched layer had athickness of 10 μm or more.

FIG. 2 is images obtained by capturing microstructures after performingnital etching on wall portions of hot press formed parts A1-1 (inventionexample) and A2-1 (comparative example) among examples. From the imagesin FIG. 2 , it can be seen that formation of ferrite is suppressed wellin the hot press formed part A1-1 in which a carbon-enriched layer wasformed in a base material surface layer part, while ferrite,deteriorating an impact property, was formed in the hot press formedpart A2-1 in which a carbon-enriched layer was not formed in a basematerial surface layer part.

FIG. 3 is graphs illustrating concentration profiles of main componentsthrough GDS analysis before and after hot press forming of A1-1(invention example). From the graphs in FIG. 1 , it can be seen that acarbon-depleted layer was not formed in a base material surface layerpart before hot press forming, while a carbon-enriched layer, having athickness of about 40 μm, was formed in a base material surface layerpart after hot press forming.

In particular, according to GDS analysis before hot press forming ofFIG. 3 , a first distance is shorter than a second distance, wherein theAl—Si plated layer comprises the first distance between a first pointand a second point; and the second distance between the second point anda third point, wherein the first point is a first intersection between afirst line and a second line, wherein the second point is a secondintersection close to the base steel sheet between a third line and afourth line, wherein the third point is where the aluminum content is 1%in GDS profile, wherein the first line is a profile of a iron content inGDS profile, wherein the second line is a profile of an aluminum contentin GDS profile, wherein the third line is 50% of an average carbonamount of the base steel sheet, wherein the fourth line is a profile ofa carbon content in GDS profile.

While examples embodiments in the present disclosure have been describedin detail, however, claims of the present disclosure are not limitedthereto, and it will be apparent to those skilled in the art thatvarious modifications and changes may be made without departing from thetechnological ideas of the present disclosure described in the claims.

The invention claimed is:
 1. A plated steel sheet for hot press formingcomprising: a base steel sheet; and an Al—Si plated layer formed on asurface of the base steel sheet, wherein GDS profiles are taken foraluminum, iron, and carbon contents of the Al—Si plated layer at asingle point, wherein the Al—Si plated layer comprises: a first distancebetween a first point and a second point; and a second distance betweenthe second point and a third point, wherein the first point is a firstintersection between a first line and a second line, wherein the secondpoint is a second intersection between a third line and a fourth lineand is closer to the base steel sheet than to a surface of the Al—Siplated layer opposite the base steel sheet, wherein the third point iswhere the aluminum content is 1% in the GDS profile, wherein the firstline is a profile of the iron content in the GDS profile, wherein thesecond line is a profile of the aluminum content in the GDS profile,wherein the third line is a line on the GDS profile indicating an amountof carbon that is 50% of an average carbon amount of the base steelsheet, wherein the fourth line is a profile of the carbon content in theGDS profile, and wherein the first distance is shorter than the seconddistance.
 2. The plated steel sheet for hot press forming of claim 1,wherein in the base steel sheet, a ratio of a carbide fraction of asurface layer part (Fs) to a carbide fraction of a central part (Fc)(Fs/Fc) is 0.7 to 1.3, and wherein the surface layer part means a regionfrom the surface of the base steel sheet to a depth of 200 μm, and thecentral part means a region having a thickness of −100 μm to +100 μmfrom a thickness center of the base steel sheet.
 3. The plated steelsheet for hot press forming of claim 1, wherein the base steel sheetcomprises by wt %, 0.15-0.4% of C, 0.05-1.0% of Si, 0.6-3.0% of Mn,0.001-0.05% of P, 0.0001-0.02% of S, 0.01-0.1% of A1, 0.001-0.02% of N,0.001-0.01% of B, 0.01-0.5% of Cr, 0.01-0.05% of Ti, and a balance of Feand inevitable impurities.
 4. The plated steel sheet for hot pressforming of claim 1, wherein the base steel sheet comprises at least oneselected from Mo, Nb, and V in such a manner that a sum thereof is 0.01to 0.5 wt %.
 5. The plated steel sheet for hot press forming of claim 1,wherein the Al—Si plated layer comprises, by wt %, 6-12% of Si, 1-4% ofFe, and a balance of A1 and inevitable impurities.
 6. The plated steelsheet for hot press forming of claim 1, wherein the Al—Si plated layerhas a thickness of 10 to 45 μm.
 7. The plated steel sheet for hot pressforming of claim 1, wherein the base steel sheet has a microstructurecomprising 10-40% of pearlite, 50-90% of ferrite, and 20% or less ofmartensite.